1. Field of Invention
The present invention relates to electro-optical devices, such as liquid crystal devices and electronic apparatuses. The present invention also relates to an electrophoresis device, such as an electronic paper, an EL (electroluminescent) device, and a device using an electron emission element (a field emission display and a surface-conduction electron-emitter display).
2. Description of Related Art
In the related art, an electro-optical device, such as a liquid crystal device capable of displaying an image, in which an electro-optical material, such as liquid crystal is interposed between a pair of substrates and light is transmitted so as to pass through the substrates and the electro-optical material, is disclosed. Displaying the image for example, is realized by changing the state of an electro-optical material in each pixel to change the transmittance ratio of light and by causing light with different gray scale levels to be sensed in each pixel.
Such an electro-optical device is active-matrix driven by including, on one of the pair of substrates, pixel electrodes arranged in a matrix, scanning lines and data lines positioned to thread through the pixel electrodes, and TFTs (thin film transistors) as pixel switching elements. According to the electro-optical device capable of being active matrix driven, the TFTs are provided to control conduction between the pixel electrodes and the data lines. The TFTs are electrically connected to the scanning lines and the data lines. Therefore, it is possible to control the turning on and off of the TFTs through the scanning lines and to apply image signals received through the data lines to the pixel electrodes, that is, to change the transmittance ratio of light in each pixel when the TFTs are turned on.
According to the above-mentioned electro-optical device, the above-mentioned various components are formed on one substrate. However, when the components are expanded two-dimensionally, the components occupy a large area. Therefore, a pixel aperture ratio, that is, a ratio of a region through which light passes to the entire surface of the substrate may decrease. Therefore, according to the related art, a method of forming the above-mentioned various components three-dimensionally, that is, a method of laminating the various components by interposing interlayer insulating layers is adopted. More specifically, the TFTs and the scanning lines functioning as gate electrode films of the TFTs are formed on the substrate. The data lines are formed on the TFTs and the scanning lines. The pixel electrodes are formed on the data lines. According to such a construction, it is possible to improve the pixel aperture ratio by appropriately arranging the various components as well as to miniaturize the electro-optical device.
At this time, the interlayer insulating films made of, for example, a silicon oxide film are formed among the above-mentioned various components so as to prevent the generation of electric short circuit among these components. Further, contact holes are formed in predetermined places of the interlayer insulating films because, for example, the drains of the TFTs must be electrically connected to the pixel electrodes and because other specific components must be electrically connected to each other. Dry etching the interlayer insulating films commonly forms the contact holes.
However, the electro-optical device having such a structure has the following problems. In general, according to the electro-optical device, it is necessary not to disturb the alignment state of the electro-optical material, such as the liquid crystal. However, according to the electro-optical device having the above-mentioned laminated structure, it is difficult to meet such a requirement. When the alignment state of the liquid crystal is disturbed, the quality of an image may deteriorate. For example, when an image, in which an entire surface is blacked, is desired to be displayed, light leakage occurs in the disturbed portion, thereby deteriorating contrast. Further, a layer contacting the liquid crystal is preferably as flat as possible in order not to disturb the liquid crystal.
Such problems occur due to the following reasons. First, the layer contacting the liquid crystal must be at the uppermost layer in the laminated structure. That is, the uppermost layer is preferably flat in order not to disturb the alignment state of the liquid crystal. However, in a case where the laminated structure is formed after the above-mentioned various components are formed, the heights of the components are different from each other. Therefore, in general, the step differences caused by the heights, overlap with each other and affect each other, toward upper layers. As a result, a complicated concavo-convex is caused in the uppermost layer. When the concavo-convex is generated, the alignment state of the liquid crystal may be disturbed in response thereto. Therefore, it is difficult to maintain the alignment state as a predetermined state. Furthermore, the deterioration of the flatness due to the above-mentioned reasons becomes more serious in response to the complexity of the structure of the electro-optical device.
Second, the contact holes formed in the interlayer insulating films also deteriorate the flatness. For example, when an alignment film is formed in the uppermost layer and also the contact holes are formed under the alignment film in the above-mentioned laminated structure, concave portions corresponding to the positions, where the contact holes are formed, are formed on the alignment film. This is because the contact hole has a hollow inside as noted from the name.
On the other hand, the light leakage mentioned as an example of the deterioration of the quality of an image may occur because the flatness of the laminated structure is not met and the contact holes themselves exist. This is because the contact hole has the hollow inside as mentioned above, thereby easily transmitting light. That is, light passing through the hollow of the contact hole is mixed with the image and then may deteriorate the quality of the image.